This invention relates generally to amplifiers and more particularly to offset adjustment circuitry in an amplifier.
Many applications require precision amplifiers. For example, data acquisition systems demand very high precision amplifiers.
One type of error in commercially available amplifiers is offset voltage. In a perfect amplifier, the output is zero when the input is zero. In a real amplifier, the output may not be precisely zero even when the input is zero. This deviation from a perfect amplifier is quantified by measuring the "offset voltage" of a real amplifier. With the output of the amplifier at zero volts, the voltage at the input is the offset voltage.
To reduce the problem of offset voltage, many commercially available integrated circuit (IC) amplifiers have pins to which a potentiometer can be connected. Adjusting the resistance of the potentiometer changes the offset voltage. If the potentiometer is properly adjusted, the offset voltage can be set very near zero or "nulled".
Several drawbacks of nulling the offset voltage with a potentiometer exist. A person usually performs the operation, which is often not convenient or cost-effective. Additionally, the offset voltage may change as the operating temperature of the amplifier changes or the amplifier ages. If the offset voltage changes, the amplifier is no longer nulled and an error is introduced.
A method of dynamic offset nulling has been used to avoid errors when the offset voltage changes. This method is called "chopper stabilization". The input to the amplifier is switched rapidly between an input signal and ground. A capacitor is connected to the output. When the input is switched to ground, the capacitor is switched between the output and ground. In this state, the output is at the offset voltage and the capacitor changes to the offset voltage. When the input to the amplifier is switched to the input signal, the capacitor is switched to be in series with the output. The capacitor is switched such that the voltage across the capacitor is subtracted from the output of the amplifier, thereby guaranteeing that the output is at zero volts when the input is at zero volts.
With chopper stabilization, the voltage across the capacitor adjusts if the offset voltage changes. Thus, the errors of adjusting the offset with a potentiometer do not occur. However, chopper stabilization has other drawbacks. Both the input and output constantly switch between the desired output signal and an undefined state. This rapid switching causes discontinuities in the output signal. The discontinuities can be filtered out, but only if the desired signal is of relatively low frequency. Additionally, the rapid switching creates high frequency noise in a system, which can be undesirable.